Lowell Observatory, astronomical observatory located in Flagstaff, Ariz.; it was founded in 1894 by Percival Lowell, the American astronomer who popularized the idea that Mars might support intelligent life. Its original telescope, still in operation, is a 24-in. (61-cm) refractor; also located at the Mars Hill site are the 13-in. (33-cm) A. Lawrence Lowell photographic camera used by Clyde Tombaugh when he discovered Pluto, and a 16-in. reflector used in the visitors' night viewing program. Located at the newer nearby Anderson Mesa station are 72-in. (183-cm), 42-in. (107-cm), and 31-in. (79-cm) reflecting telescopes and a 24-in. (60-cm) Schmidt telescope used in the search for asteroids and other near-earth objects. Anderson Mesa is also the site of the Navy Prototype Optical Interferometer program, a joint venture of the Lowell Observatory, the U.S. Naval Observatory, and the Naval Research Laboratory. Many discoveries of fundamental importance have been made by the observatory, especially by V. M. Slipher, its director from 1916 to 1954. By 1917 he had determined through spectroscopic analysis the radial velocities of most spiral nebulae then known. His discovery that nearly all these nebulae, now known as galaxies, were apparently moving away from the earth led to Edwin Hubble's work and the discovery of the expanding universe. Beginning in 1905 the observatory made a concerted search for a transneptunian planet, which led to Tombaugh's discovery of Pluto in 1930. Principal research programs involve the discovery and determination of orbits for new asteroids, a search for nearby stars, and the measurement of light and motion of close double stars, nebulae, and other galactic objects.